Bone marrow edema (BME) represents a reversible but highly painful finding in magnetic resonance imaging (MRI) of patients with knee osteoarthritis. The aim of this retrospective study was to evaluate the efficacy of extracorporeal shock wave treatment (ESWT) on painful BME in osteoarthritis of the knee. This study focuses on people who had early-to-mid stage osteoarthritis with knee pain and MRI findings of BME. Patients who underwent ESWT treatment or prescribed alendronate treatment in our department were analyzed. Knee pain and function were measured using the visual analog scale (VAS) for pain and the Western Ontario and McMaster University Osteoarthritis Index (WOMAC), respectively. The degree of BME was measured with MRI scans. A total of 126 patients who received ESWT treatment (Group A, n=82) or alendronate treatment (Group B, n=44) were included. All patients were followed up clinically and radiographically for a minimum of 12 months. The mean follow-up was 23.5 months (range, 12-38 months). The VAS and WOMAC score decreased more significantly after treatment in Group A than that in Group B (P<.01) within 3 months. In 6-month MRI follow-ups, there was higher incidence of distinct reduction and complete regression of BME of the affected knee in Group A than that in Group B (P<.01). ESWT is an effective, reliable, and noninvasive treatment in patients with painful BME in osteoarthritis of the knee followed by a rapid normalization of the MRI appearance. It has the potential to shorten the natural course of this disease.